This invention relates to machine rotor structures and more particularly to turbine rotor disc assemblies.
In many instances involving large axial flow elastic fluid utilizing machines, such as steam turbines, the rotor structure is of such a large size that at least some of the blade carrying discs and the rotor shaft are formed separately and assembled by shrink fitting the discs onto the rotor shaft. Stress corrosion cracking can occur at the disc bore-rotor shaft interface of large shrunk-on, low pressure steam turbine discs. In a typical design, these discs are attached to the rotor shaft by a heat shrinking and keying operation. For example, after heat shrinking, three axial keyways may be drilled at equally spaced locations in the disc shaft interface. Keys inserted in these keyways serve to lock the disc onto the shaft and also to transmit torque in case of a loss in shrink fit. Crevices formed by the keys in the keyways become a primary location for the development of stress corrosion cracking. This can result in cracking at the keyways of the shrunk-on discs. Crevices formed between the disc and the rotor shaft can also result in cracks in the bore of the rotor discs.
Stress corrosion cracking requires the presence of an agressive environment, high yield strength material and tensile stresses above some threshold value. The reduction of any of these factors can reduce susceptibility to stress corrosion cracking. Consequently, the elimination of stress risers and the introduction of compressive stresses in the bore of the disc can reduce the disc bore tensile stress below the threshold value. The present invention provides a method of rotor disc structure assembly wherein compressive stresses are inducted in a sleeve and the bore of the disc and the assemblies are then attached to the rotor shaft without the introduction of stress risers in the discs.
A rotor disc assembly constructed in accordance with the present invention comprises: a disc having an axial bore; a sleeve positioned within the axial bore and forming an interference fit with the axial bore wherein the sleeve and disc bore are subjected to compressive stress; and a rotor shaft passing through the sleeve and having a driving connection therewith. The method for constructing this rotor disc structure comprises the steps of: placing a sleeve within a bore in a disc to form a clearance fit between the outside surface of the sleeve and the inside surface of the bore; plastically deforming the sleeve and disc bore to place the sleeve and disc bore in compression; and mounting the assembled disc and sleeve onto a rotor. Before placing the sleeve within the disc bore, the bore may be polished to remove residual tensile stress. The plastic deformation of the sleeve and bore is accomplished by: heating the assembled disc and sleeve to increase the inside diameter of the sleeve; inserting a plug of material having a higher coefficient of thermal expansion than the disc and sleeve, and having an outside diameter larger than the original inside diameter of the sleeve, into the sleeve; heating the assembled plug, disc and sleeve, to plastically deform the sleeve and disc bore; cooling the assembled plug, disc and sleeve, to create compressive stresses within the sleeve and disc bore; and removing the plug. In order to facilitate plug removal, the sleeve may be provided with a taper bore which matches a tapered outer surface of the plug. In that case, the sleeve will be machined to a predetermined internal diameter before being mounted on the rotor shaft. In addition to the interference fit between the disc and the sleeve, the sleeve may also be provided with a turn which is eccentric to its bore which fits into a corresponding eccentric portion of the disc bore to prevent rotation of the disc on the sleeve due to torque being transmitted by the disc to the sleeve. After the disc and sleeve are assembled onto the shaft, the sleeve may be keyed to the shaft in accordance with known techniques. The keyways will not be susceptible to stress corrosion cracking since the stress risers will be in the sleeve, which during operation, will have low tensile stresses. As the rotor shaft assembly reaches operating speed, the disc bore will increase in diameter due to the decrease in bore compressive stresses. Since the compressive stress in the sleeve is also reduced, the sleeve will increase in diameter, thereby maintaining the induced interference fit, and its ability to transmit torque.